Liquid-phase oxidation of an aromatic alkyl to an aromatic carboxylic acid is a highly exothermic chemical reaction. Volatile aqueous acidic solvents are typically used to contain the reaction mixture and to dissipate the heat of reaction. The oxidation of aromatic alkyls is generally performed with a mixture of an ionic catalyst, such as cobalt, manganese, and bromine. Preferred ionic catalysts are cobalt acetate, manganese acetate hydrobromic acid, cobalt and manganese bromide, or any other form that makes available the ionic forms of these elements in the reaction medium.
The preferred oxidation solvent is a low molecular weight aliphatic mono-carboxylic acid having 2 to 6 carbon atoms, inclusive, or mixtures thereof with water. An example of such a solvent is acetic acid, or mixtures of acetic acid and water. Some U.S. patents relate to processes using other solvents, for example, U.S. Pat. Nos. 4,357,475, 4,892,970, and 5,760,288. A reaction temperature of 145° C. to 235° C. is typical, and the reaction pressure is such that the reaction mixture is kept under liquid phase conditions. A promoter such as a low molecular weight ketone having 2 to 6 carbon atoms or a low molecular weight aldehyde having 1 to 6 carbon atoms can also be used. Bromine promoter compounds known in the art, such as hydrogen bromide, molecular bromine, sodium bromide and the like can also be used. A source of molecular oxygen is also used for the conversion of p-xylene to terephthalic acid. It can vary in molecular oxygen content from that of 10% molecular oxygen, to oxygen gas, as shown for example in U.S. Pat. Nos. 5,371,283, 6,175,038 B1, 6,180,822 B1, and 6,194,607 B1.
Air is the preferred source of molecular oxygen. In order to avoid the formation of explosive mixtures, however, the oxygen-containing gas fed to the reactor should provide an exhaust gas-vapor mixture containing from 0.5 to 8 volume percent oxygen (measured on a solvent-free basis). For example, a feed rate of the oxygen-containing gas sufficient to provide oxygen in the amount of from 1.5 to 2.8 moles per methyl group will provide such 0.5 to 8 volume percent of oxygen, measured on a solvent-free basis, in the overhead gas-vapor mixture, as noted in U.S. Pat. Nos. 2,962,361 and 3,155,718.
U.S. Pat. No. 3,839,435 (the “'435 patent”) relates to ways of feeding a reaction mixture and the location of the delivery of the reaction mixture into an oxidation reactor. A reactor comprising four levels of Rushton-type agitator is shown wherein reactants are fed through separate feeding pipes to zones close to the impellers. As recited in the '435 patent, p-xylene is fed through pipes 3a, 3b and 3c to a zone near the upper edge of the first, second and third levels of impellers; the mixture of solvent and catalyst is fed through pipe 4 to a point in the upper half portion of the reactor and the recycled stream of condensed solvent is fed through pipe 10 to a point in the lower half of the reactor. The oxygen containing gas is introduced through pipe 5 to the bottom of the reactor below the lowest level of impellers so that the gas bubbles ascend through the liquid phase reactants. The '435 patent notes that the quality of the terephthalic acid produced is influenced by the manner of supplying p-xylene into the reaction zone; and, therefore, the '435 patent proposes to feed p-xylene through at least three inlets located and distributed in the vertical direction inside the reactor, and that it is desirable to supply the p-xylene through the inlets as near as possible to the end of the blades of a stirrer. The '435 patent, however, requires at least three feeding pipes for only one reactant, which results in a complex configuration.
U.S. Pat. No. 4,062,654 relates to a reaction vessel for producing an aromatic carboxylic acid by liquid-phase oxidation with a molecular oxygen-containing gas, wherein solvent is sprayed by means of a porous plug or a perforated ring to prevent the adhesion of the aromatic carboxylic acid to the inside wall of the reactor. The materials are fed to the reactor at non-specific points located above a one stage impeller. There is no teaching in this patent about specific feed locations to improve the conversion efficiency.
U.S. Pat. No. 4,159,307 relates to an apparatus for producing an aromatic dicarboxylic acid by oxidizing a benzene derivative with molecular oxygen in the liquid phase, wherein the starting material is fed into the liquid phase in a uniformly dispersed state by being passed through a porous material causing the pressure of the starting material liquid do to drop more than 1 Kg/m2. There is no teaching in this patent about specific feed locations to improve the conversion efficiency.
U.S. Pat. No. 4,243,636 relates to an oxidation reactor comprising tooth-like stirring elements in the form of comb-like agitating blades. The reacting mixture is fed through a pipe to a point located at the bottom of the reactor and the product is withdrawn through a pipe located at the upper portion of the reactor. There is no teaching in this patent about specific feed locations to improve the conversion efficiency, but is addressed to the form of the agitating blades.
It is desirable, therefore, to provide a process and an apparatus for the conversion of an aromatic alkyl to a carboxylic acid in an oxidation reactor, wherein the reactor has a simple and low-cost design. None of the above documents discloses the combination of features and advantages of the present invention.